"No, Air Force One over here. Used to be more commonplace for the Prime Minister to fly aboard an RAF VC10, though now it is more BA 777, occassionally Concorde when it was necessary and flying of course"

Due to security concerns, something similar but different to Air Force 1 has occasionally been mooted for times of crisis. Last I read, an RAF aircraft of some type but very modular inside to suite a variety of situations.

The problem with this? The spin doctors fear it might be dubbed Blair Force One and put a dent in the PM's popularity. See:

http://news.bbc.co.uk/1/hi/uk_politics/2978756.stm

http://news.bbc.co.uk/1/hi/uk_politics/2980910.stm

Personally, I think the PM seen flying BA is good for BA and generally good for the national image.